This invention relates to a catalyst composition having as components zinc oxide, copper oxide and aluminum oxide in combination with Y zeolite and clay suitable for the conversion of syngas, a mixture of hydrogen and carbon monoxide, to dimethyl ether and to the process for converting synthesis gas (syngas) to dimethyl ether employing this catalyst. This invention also relates to two-step processes for converting syngas to light olefins by contacting the effluent from a conversion of syngas employing a catalyst composition having as components zinc oxide, copper oxide and aluminum oxide in combination with Y zeolite and clay with a second catalyst system having a SAPO component such as, in particular, SAPO-34 or modified SAPO-34. This invention is further related to catalyst systems useful in the conversion of syngas to light olefins in two step processes.
Catalysts having zinc oxide, copper oxide and aluminum oxide as components are known to be effective as methanol synthesis catalysts. Catalysts having as components zinc oxide, copper oxide and aluminum oxide or chromium oxide in conjunction with a gamma aluminum oxide are known to effect the conversion of syngas to dimethyl ether. The present invention provides an improved conversion and selectivity to dimethyl ether by replacing gamma aluminum oxide with a combination of Y zeolite and clay in a catalyst system useful for the conversion of syngas.
Processes for converting methanol and its derivatives, including dimethyl ether, to light olefins employing a SAPO catalyst are also known. U.S. Pat. No. 4,499,327 discloses the preparation of light olefins from dimethyl ether or methanol employing a SAPO catalyst, but does not disclose the preparation of dimethyl ether using the process cited above.
There is a growing interest in alternative liquid fuels produced from natural gas. Dimethyl ether is a clean and efficient alternative diesel fuel which can be produced by the dehydration of methanol which can be synthesized from syngas. There is a significant economic advantage in integrating the process for producing dimethyl ether from syngas into a single process. There is also a continued expansion of the uses to which light olefins can be put and therefore a commercial reward for improving the methods by which light olefins can be produced. Syngas has proved to be an attractive source from which light olefins can be obtained. Accordingly, development of these improved processes for converting syngas to dimethyl ether or light olefins and development of the catalyst systems useful in these processes would be a significant contribution to the art.
It is an object of this invention to provide an improved method for converting syngas to dimethyl ether. It is another object of this invention to use this improved process in two step processes for the conversion of syngas to light olefins. Still another object of this invention is to employ SAPO in the second step of converting dimethyl ether to light olefins in these two step processes for converting syngas to light olefins. Another object of the invention is to provide methods for converting dimethyl ether to light olefins that show improved results employing modified SAPO-34 as compared to the use of SAPO-34 in a two step process. A yet further object of this invention is to provide the catalyst systems useful in the conversions set out above. Other objects and advantages will be apparent from the detailed description and the appended claims.
In accordance with an embodiment of this invention a method is provided for converting syngas by contacting syngas under conversion conditions with a catalyst comprising zinc oxide, copper oxide, aluminum oxide, Y zeolite and clay.
In accordance with another embodiment of this invention a method is provided for the conversion of syngas to dimethyl ether (DME) by contacting syngas under conversion conditions with a catalyst comprising zinc oxide, copper oxide, aluminum oxide, Y zeolite and clay.
In accordance with still another embodiment of this invention a method is provided for the conversion of syngas to light olefins by contacting syngas under conversion conditions with (A) a first catalyst comprising zinc oxide, copper oxide, aluminum oxide, Y zeolite and clay under conditions for the conversion of syngas to dimethyl ether and then contacting the effluent from contact with the first catalyst with (B) a second catalyst comprising SAPO under conditions for the conversion of dimethyl ether to light olefins.
In accordance with other embodiments of this invention methods are provided for the conversion of syngas to light olefins in which the second catalyst is SAPO-34 and (1) the temperature of the second step is increased as compared to the temperature of the first step or (2) the temperature of the second step is increased as compared to the temperature of the first step and the pressure of the effluent from the contact with the first catalyst is reduced before contact with the second catalyst.
In accordance with other embodiments of this invention methods are provided for the conversion of syngas to light olefins in which the second catalyst is a modified SAPO-34 such as SAPO-34 modified with lanthanum(III) nitrate hexahydrate; SAPO-34 modified with magnesium hydrate hexahydrate; SAPO-34 modified with tributyl borate or SAPO-34 modified with triethyl phosphate.
Other objects and advantages of the invention will become apparent from the detailed description and the appended claims.
For the purposes of this disclosure the citing in the specification, claims or abstract of a component or components should be construed as including a restrictive phrase chosen from among the following: in the broadest sense as not excluding the presence of other components, i.e. xe2x80x9ccomprisingxe2x80x9d; in a more restricted sense as containing only the material cited as essential to the invention with no other material present in an amount to affect the invention, i.e. xe2x80x9cconsisting essentially ofxe2x80x9d, and in the most restricted sense as the presence of only the material cited, i.e. xe2x80x9cconsisting ofxe2x80x9d. The use of any of the quoted phrases limits the disclosed component or components in the sense defined above.
Synthesis gas (syngas) is a gaseous mixture of hydrogen and at least one carbon oxide, particularly carbon monoxide. Syngas is obtained using well known processes by the partial combustion or gasification of any organic material such as coal, other hydrocarbons, carbohydrates and the like. It is well known that Syngas can be subjected to a heterogeneous catalytic reaction using a copper-based catalyst such as copper oxide, zinc oxide and aluminum oxide to produce methanol. This is the same catalyst that is used in this invention along with Y zeolite and clay to produce dimethyl ether.
The Y zeolite useful in the production of dimethyl ether is well known and a typical synthetic zeolite usually prepared in the sodium form, that is, with a sodium cation in close proximity to each aluminum tetrahedron thereby balancing its charge. Type Y zeolite has a ratio of Al2O3 to SiO2 in the range of about Al2O3/3.0-6.0 SiO2 and an effective pore size of approximately 7.4 Angstroms.
In the integrated process for producing dimethyl ether from syngas a commercial methanol catalyst, having copper oxide, zinc oxide and aluminum oxide as components is combined with Y zeolite, a clay component, preferably Bentonite, and sufficient water to form a paste which is then extruded, dried and then calcined to provide a catalyst mixture. The copper oxide is present in a range of about 20 percent by weight to about 40 percent by weight, preferably about 25 percent by weight to about 35 percent by weight. The zinc oxide is present in a range of about 5 percent by weight to about 25 percent by weight, preferably about 10 percent by weight to about 20 percent by weight. The aluminum oxide is present in a finite range of up to about 15 percent by weight, preferably up to about 10 percent by weight. The Y zeolite is present in a range of about 1 percent by weight to about 20 percent by weight, preferably about 5 percent by weight to about 15 percent by weight. The clay is present in a range of about 30 percent by weight to about 50 percent by weight, preferably about 35 percent by weight to about 45 percent by weight.
Generally water is added in an amount sufficient to produce a paste. The paste is thoroughly mixed and extruded. The extrudate is dried at a temperature in the range of about 90xc2x0 C. to about 150xc2x0 C., preferably in a range of about 100xc2x0 C. to about 140xc2x0 C., and most preferably about 110xc2x0 C. to about 130xc2x0 C. for a time in the range of about 1 to about 5 hours, more preferably about 2 to about 4 hours. The dried extrudate is then calcined at a temperature in the range of about 300xc2x0 to about 400xc2x0 C., preferably about 325xc2x0 C. to about 375xc2x0 C. for a time in the range of about 1 to about 5 hours, preferably about 2 to about 4 hours.
In the reaction to convert syngas to dimethyl ether a feed gas having about 15 to about 35 volume percent CO, about 1 to about 10 volume percent CO2 and about 60 to about 80 volume percent H2 at a mole ratio of H2:CO in a range of about 1.5 to about 4.0, preferably about 2.0 to about 3.8 is contacted in the presence of a calcined catalyst composition of an extruded mixture of copper oxide, zinc oxide, aluminum oxide, Y zeolite and clay under conversion conditions of reaction temperature in the range of about 240xc2x0 C. to about 300xc2x0 C., preferably about 260xc2x0 C. to about 280xc2x0 C. at a pressure of about 300 psig to about 1500 psig, preferably from bout 400 psig to about 1000 psig and a feed rate in a range of about 50 cc/min to about 400 cc/min.
The effluent from the reaction to convert syngas to dimethyl ether can be fed to a second reaction zone in which the catalyst is based on a silicoaluminophosphate (SAPO) material. SAPO catalysts exhibit properties of both aluminosilicate zeolites and aluminophosphates. SAPO material has a three dimensional microporous crystal framework structure of PO2, AlO2 and SiO2 tetrahedral units. The chemical composition (anhydrous) is: mR:(SixALyPz)O2 wherein xe2x80x9cRxe2x80x9d represents at least one organic templating agent present in the intracrystalline pore system; xe2x80x9cmxe2x80x9d represents the moles of R present per mole of (SixALyPz)O2 and has a value of from zero to 0.3, the maximum value depending on the molecular dimensions of the templating agent and the available void volume of the pore system of the particular SAPO species involved, and the xe2x80x9cxxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d represent the mole fractions of silicon, aluminum and phosphorus, respectively. Further details of the formation of SAPO compositions can be found in U.S. Pat. No. 4,440,871, the entire disclosure of which is expressly incorporated herein by reference.
Examples of such templating agents include, but are not limited to, tetramethylammonium hydroxide, tetraethylammonium hydroxide and tetrapropylammonium hydroxide. Further details relating to the formation of SAPO compositions, including molar amounts of each oxide source, can be found in Lok et al., U.S. Pat. No. 4,440,871, the entire disclosure of which is expressly incorporated herein by reference.
Among the SAPO compositions useful in the present invention are SAPO-4, SAPO-5, SAPO-11, SAPO-16, SAPO-17, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-37, SAPO-40, SAPO-41, SAPO-42 and SAPO-44. Most preferred at present is SAPO-34
The SAPO compositions can be chosen from the silicoaluminaphosphate materials per se or these materials can be modified by the inclusion of other chemical entities found effective to enhance the conversion of CO in the syngas or to affect the selectivity of the product to increase the amount of desired light olefins. Among the materials that have been determined to effect these results when combined with the SAPO compositions are lanthanum(III) nitrate hexahydrate, magnesium hydrate hexahydrate, tributyl borate and triethyl phosphate. Each of these materials can be combined with the SAPO compositions by admixing in the presence of a suitable liquid carrier which can be chosen from water and various organic liquids such as n-hexane or cyclohexane to form a paste. After thorough mixing the paste formed can be dried and later calcined into a particulate catalyst.
In general the amounts of compounds added to the SAPO compositions are in the range of about 2.0 percent by weight lanthanum to about 3.0 percent by weight lanthanum; about 1.5 percent by weight magnesium to about 2.5 percent by weight magnesium; about 3.5 percent by weight to about 4.5 percent by weight phosphorus and about 4.5 percent by weight to about 5.5 percent by weight boron. The drying is carried out at an elevated temperature of up to about 150xc2x0 C., preferably up to 120xc2x0 C. for a time of about 1 to about 5 hours, preferably about 2 to about 4 hours. The calcining is carried out at an elevated temperature of up to about 600xc2x0 C., preferably up to about 550xc2x0 C. for a time in the range of about 1 to about 5 hours, preferably about 2 to about 4 hours.
The further conversion of the effluent from contact with a first catalyst bed having as components copper oxide, zinc oxide and aluminum oxide with a combination of Y zeolite and a clay is carried out by passing the effluent from the first catalyst bed through a second catalyst bed having the above described SAPO or modified SAPO components. In general the reaction conditions for the reaction with the second catalyst composition are a flow rate in the range of about 50 cc/min to about 200 cc/min, a reaction temperature in the range of about 300xc2x0 C. to about 650xc2x0 C., preferably about 350xc2x0 C. to about 600xc2x0 C. at a pressure in the range of about atmospheric up to about 350 psig, preferably up to about 300 psig. It has been found that one of the preferred means for increasing the selectivity to light olefins is to lower the pressure on the effluent from the first reaction bed before the effluent enters the second reaction bed. Lowering this pressure to atmospheric has been found very effective to increase the selectivity to light olefins in the second reaction.